(1) Field of the Invention
This invention relates to an optical transmission apparatus and, more particularly, to an optical transmission apparatus for transmitting an optical pulse signal.
(2) Description of the Related Art
Wavelength division multiplexing (WDM) multiplexes a number of optical signals with different wavelengths into a single optical fiber. By doing so, a plurality of signals are transmitted at the same time by one optical fiber. WDM is a core technique for an optical transmission system and has been developing rapidly.
FIG. 14 shows the structure of a WDM system. A WDM system 200 comprises an optical sending section 210 and an optical receiving section 220. The optical sending section 210 includes transponders 211-1 through 211-n, optical attenuators (ATT) 212-1 through 212-n, a MUX section 213, and an optical amplifier 214. The optical receiving section 220 includes an optical amplifier 221, a DMUX section 222, and transponders 223-1 through 223-n. 
In the optical sending section 210, the transponders 211-1 through 211-n correspond to wavelengths λ1 through λn respectively. The transponders 211-1 through 211-n convert the wavelength bands of input optical signals into narrow wavelength bands suitable for WDM and output the input optical signals. The ATTs 212-1 through 212-n control the optical power of the input optical signals with the wavelengths λ1 through λn and send them to the MUX section 213 at the next stage by which multiplexing is performed. The MUX section 213 multiplexes the wavelengths λ1 through λn to generate a WDM signal (wavelength multiplexed signal). The optical amplifier 214 amplifies the WDM signal and outputs it onto an optical transmission line.
In the optical receiving section 220, the optical amplifier 221 receives and amplifies the WDM signal sent via the optical transmission line. The DMUX section 222 separates the WDM signal into the wavelengths λ1 through λn. The transponders 223-1 through 223-n correspond to the wavelengths λ1 through λn respectively. The transponders 223-1 through 223-n convert the narrow bands of the optical signals after the separation into wavelength bands suitable for units on the user side and output them.
The number of the above transponders required for converting an interface between user units and WDM is equal to that of wavelengths which can be handled in the WDM system. Traditionally, the number of transponders which must be located also increases according to the number of kinds of line speeds which can be handled in a WDM system.
For example, it is assumed that the WDM system 200 provides a service at line speeds of 600 Mb/s and 2.4 Gb/s. Then the transponders 211-1 through 211-n corresponding to a line speed of 600 Mb/s, the transponders 211-1 through 211-n corresponding to a line speed of 2.4 Gb/s, the transponders 223-1 through 223-n corresponding to a line speed of 600 Mb/s, and the transponders 223-1 through 223-n corresponding to a line speed of 2.4 Gb/s must be located. That is to say, with the transponder 211-1 for converting the wavelength λ1, two boards corresponding to line speeds of 600 Mb/s and 2.4 Gb/s, respectively, must be used. The same applies to the other transponders.
As a result, a WDM system which provides a service at line speeds of 600 Mb/s and 2.4 Gb/s by using the n wavelengths λ1 through λn requires 2n transponders at the sending end. That is to say, this system requires n transponders which correspond to the n wavelengths λ1 through λn, respectively, and which correspond to a line speed of 600 Mb/s and n transponders which correspond to the n wavelengths λ1 through λn, respectively, and which correspond to a line speed of 2.4 Gb/s at the sending end. Similarly, this system requires 2n transponders at the receiving end. Accordingly, this system includes a total of 4n transponders. Conventionally, the number of transponders increases in this way not only according to the number of wavelengths used but also according to the number of kinds of line speeds used. Therefore, each time a function or the like is added, a great deal of effort is spent in maintaining and managing the system.
In recent years a bit free transponder which can convert the wavelength bands of input optical signals without depending on a transmission line speed in the synchronous digital hierarchy (SDH), the synchronous optical network (SONET), the gigabit Ethernet (Ethernet is a registered trademark) (GbE), or the like has been developed.
If such bit free transponders are used, there is no need to use boards according to line speeds. The number of the bit free transponders used should correspond to that of wavelengths used. (In the above example, one bit free transponder can handle line speeds of 600 Mb/s and 2.4 Gb/s. Accordingly, the number of bit free transponders used should correspond to that of the wavelengths used. That is to say, a total of 2n bit free transponders should be located at the sending and receiving ends.) However, there is no limit to the line speed of a signal sent from a user. Accordingly, control must be exercised in some way to maintain communication charges first settled by contract.
A technique for connecting a user LAN and the wide area LAN of a common carrier via an optical fiber transmission line and for band-limiting the amount of data according to a band settled under contract with a user has conventionally been proposed as a technique for limiting a signal from a user (see, for example, Japanese Unexamined Patent Publication No. 2002-94574 (paragraphs [0009]-[0030] and FIG. 1)).
The use of the above-mentioned bit free transponders obviates work, such as replacing boards or evaluating them one by one, which must otherwise be performed each time a line speed is changed. Therefore, a maintainer or a manager can manage a system efficiently.
However, if a bit free transponder which can relay signals sent at various line speeds is used, a guard mechanism by which a signal sent at a certain line speed cannot be transmitted is needed. If such a guard mechanism is not used, communication charges first settled under contract with each user cannot be maintained because a communication charge system is determined on the basis of line speed from the viewpoint of each user who makes use of a communication system. As a result, the communication system cannot be operated properly.
That is to say, if a conventional transponder without the function of a bit free transponder is used and a line speed of 600 Mb/s is settled first under contract with a user, then a transponder corresponding to a line speed of 600 Mb/s is located in the system. Accordingly, even if the user attempts to perform high-speed communication at a line speed higher than 600 Mb/s, the user cannot communicate. As a result, the system is operated properly in accordance with the initial contract.
With a system in which a bit free transponder is located, however, band conversion can be performed regardless of line speed. Accordingly, even if a line speed of 600 Mb/s is settled under contract with a user, a signal sent at a higher line speed (2.4 Gb/s, for example) can be transmitted by the communication function of the system regardless of the line speed settled by contract.
Therefore, with a system using bit free transponders without a guard mechanism, problems arise. For example, a user who made a low-speed line contract can use a high-speed line if the user does not inform about it (a user who made a low-speed line contract with malice may utilize a high-speed line service). In such a case, communication charges first settled under contract with the user cannot be maintained, resulting in a deterioration in the reliability of the operation of the system and the quality of services.
With the above conventional technique (Japanese Patent Laid-Open Publication No. 2002-94574), on the other hand, an optical signal sent from a user is converted into an electrical signal. The band of a user signal as an electrical signal is limited by discarding frames outside the band settled by contract. However, most of current WDM systems process optical signals transmitted without converting them into electrical signals. Accordingly, the technique which performs band limitation in the above way cannot be applied to such systems.